The present invention relates to catalytically active coatings on the surfaces of moulded shaped bodies for the synthesis of hydrogen cyanide from hydrocarbons and ammonia, wherein the catalytically active layer contains nitrides and at least one platinum metal. More particularly, the present invention relates to the process of making these catalysts. In another aspect, the present invention relates to the use of the catalyst produced as described above for the synthesis of hydrogen cyanide from hydrocarbons and methane.
According to the so-called BMA process developed by Degussa A. G., lower hydrocarbons, in particular methane, are reacted with ammonia at temperatures of about 1000 to 1350.degree. C. in the presence of a catalyst to give hydrogen cyanide (HCN) and hydrogen (see Ullmann's Encyclopedia of Industrial Chemistry, 5th edition 1987, vol. A 8, pages 162-163 incorporated herein by reference). The strongly endothermic reaction generally takes place in tubular reactors. The internal surface of the reaction tubes, substantially made from aluminum oxide, which are suspended in a BMA tubular reactor and are externally heated, is provided with a catalytically active coating.
According to the process in DE-A 10 13 636, the reaction gases are passed over catalysts which, in addition to one or more platinum group metals, in particular platinum, also contain aluminum or elements from the lanthanide series, individually or several together, substantially in the form of their nitrides. A preferred catalytically active coating on moulded bodies made of aluminum oxide contains platinum and aluminum nitride.
To prepare this type of coating, the moulded bodies are impregnated with, for example, aluminum-containing solutions of hexachloroplatinic acid. After drying, the catalytically active components are reduced with hydrogen at 600 to 900.degree. C. The nitride is formed in the BMA reactor during the start-up phase, under the effect of the ammonia used for HCN synthesis. The start-up phase, only after which does the catalyst achieve its full potential, lasts about 25 hours.
A substantially improved process for preparing the catalytically active coating is described in DE 39 23 034 C2. According to this patent, the surfaces of the moulded bodies are wetted with a dispersion which contains, as coating components, particulate elemental platinum groups metal and particulate elemental aluminum metal with the particle sizes of each being substantially less than 100 .mu.m. The surfaces of the moulded bodies treated in this way are slowly heated up to the reaction temperature of the BMA process, 1000 to 1350.degree. C., in the presence of nitrogen and/or ammonia in order to convert the coating into the catalytically active state thereby forming the catalyst needed for the chemical reaction producing the hydrogen cyanide. If ammonia is used during the heating procedure, then the catalyst has been effectively formed on achieving the reaction temperature.
An object of the present invention is to provide a process for preparing catalytically active coatings on the surface of moulded bodies substantially made from aluminum oxide, for the synthesis of hydrogen cyanide, which provide catalytically active coatings having a higher catalytic activity than conventional coatings and capable of being loaded in a very short time with the high reactant flow-rates present during steady-state operation.